Firefox by default supports image resize inside wyswiwyg editor.
For other browsers you can use jQuery UI's resizable feature.
An example is included in wysiwyg demo page:
mustache/app/views/assets/scripts/wysiwyg.js
Search for enableImageResize function inside it

Markdown Editor

To use markdown editor you should include
assets/js/markdown/markdown.js
and
assets/js/markdown/bootstrap-markdown.js
and optionally
assets/js/bootbox.js
if you prefer bootstrap modals to default browser dialogs

For better styling you should put it inside a widget box

By specifying data-provide="markdown" attribute for the textarea element,
markdown editor will automatically be initiated.